Membranes serve to delineate the boundaries of living cells and proteins incorporated into membranes help to define their biological functions. The relevance of bacterial membrane proteins is exemplified by the fact that nearly a third of all bacterial proteins produced are membrane-associated or secreted. Streptococcus mutans is a cariogenic oral pathogen whose virulence is determined in large part by its membrane composition. Human dental caries is a ubiquitous infectious disease that represents 5-10% of healthcare expenditures in industrialized nations. The Signal Recognition Particle (SRP) pathway of Streptococcus mutans is known to play an important role in its membrane biogenesis and pathogenesis. A further understanding of this pathway is necessary to understand the basic underlying machinery by which S. mutans virulence factors get into or through the cytoplasmic membrane. S. mutans SRP mutants demonstrate growth defects, are unable to contend with acid, osmotic, or oxidative stress, and demonstrate significantly decreased cariogenicity in a murine model. The SRP pathway is a co-translational protein translocation system that is conserved in all living cells. In bacteria three conserved components are recognized: Ffh, a protein with GTP hydrolysis (GTPase) activity that binds nascent polypeptides; FtsY, a membrane bound receptor that binds Ffh and helps target nascent peptides to the translocon channel; and a small cytoplasmic RNA (scRNA) that is involved in the stability of Ffh-FtsY and catalysis of GTPase activity. This proposal will investigate YlxM, a previously uncharacterized protein whose gene is found immediately upstream of ffh in S. mutans and other Gram- positive and some Gram-negative organisms. Based on preliminary data presented in this application YlxM is hypothesized to represent an additional component of the S. mutans SRP pathway. Three aims are proposed: 1) To confirm and characterize the interaction of YlxM with Ffh; 2) To confirm and characterize the interaction of YlxM with the scRNA; and 3) Evaluate the influence of YlxM on the function of SRP pathway components. This research supports the NIDCR goal to investigate cariogenic organisms to identify genes and pathways that contribute to the decay process. Additionally this research represents the basis of a doctoral dissertation project. Hence, this fellowship application will assist in trainin a student with an interest in oral health research to become an independent investigator.